Many and various types of drum mixers for making asphaltic concrete have been known, proposed and/or used in the past. One such drum mixer has an inclined drum rotatable about its axis and having an inlet at its upper end and an asphaltic concrete product outlet at its lower end. A burner assembly extends through the breeching at the lower end of the drum and mounts a burner head spaced from the lower drum end to define a drying chamber between the burner head and the aggregate inlet and an annular mixing chamber between the burner head and the asphaltic concrete product outlet. Flighting is typically spaced circumferentially about and longitudinally along the interior wall of the drying section of the drum to carry and veil aggregate input to the drum at its upper end. Hot gases of combustion from the burner flame flow in the drying section concurrently to the direction of flow of the aggregate through the drum, the hot gases flowing through the veiling aggregate to remove dust and moisture from the aggregate and exiting the drum for flow to a separator, e.g., a baghouse.
The mixing section comprises the annular chamber between the burner tube assembly projecting into the drum from its lower end wall and the drum walls. A liquid asphalt pipe extends into the mixing chamber for discharging liquid asphalt onto the dried aggregate flowing into the mixing chamber whereby the dried aggregate and asphalt form asphaltic concrete.
Used or recycled asphaltic product and/or mineral filler, dust and/or additives may also be disposed in the drum for mixing in the mixing chamber with the aggregate whereby the asphaltic concrete comprises virgin aggregate and recycled asphaltic product and with/or without applied liquid asphalt. Drums of this type have been successfully used for many years in the asphalt industry. A recurring and constant problem whenever asphalt and hot gases or flame are used in conjunction with one another is the "blue smoke" generated by the burning and/or volatilization of the asphalt. This causes environmental problems and it is therefore highly desirable to eliminate any production of blue smoke. Additionally, efficiencies in heating the virgin aggregate, as well as the recycled asphaltic product, are also highly desirable in view of the large tonnage of materials passing through the mixer.